Simple way to repeat a string

Here is the shortest version (Java 1.5+ required):

repeated = new String(new char[n]).replace("\0", s);

Where n is the number of times you want to repeat the string and s is the string to repeat.

No imports or libraries needed.

Commons Lang StringUtils.repeat()

Usage:

String str = "abc";
String repeated = StringUtils.repeat(str, 3);

repeated.equals("abcabcabc");

So you want to avoid loops?

Here you have it:

public static String repeat(String s, int times) {
    if (times <= 0) return "";
    else return s + repeat(s, times-1);
}

(of course I know this is ugly and inefficient, but it doesn't have loops :-p)

You want it simpler and prettier? use jython:

s * 3

Edit: let's optimize it a little bit :-D

public static String repeat(String s, int times) {
   if (times <= 0) return "";
   else if (times % 2 == 0) return repeat(s+s, times/2);
   else return s + repeat(s+s, times/2);
}

Edit2: I've done a quick and dirty benchmark for the 4 main alternatives, but I don't have time to run it several times to get the means and plot the times for several inputs... So here's the code if anybody wants to try it:

public class Repeat {
    public static void main(String[] args)  {
        int n = Integer.parseInt(args[0]);
        String s = args[1];
        int l = s.length();
        long start, end;

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatLog2(s,i).length()!=i*l) throw new RuntimeException();
        }
        end = System.currentTimeMillis();
        System.out.println("RecLog2Concat: " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatR(s,i).length()!=i*l) throw new RuntimeException();
        }               
        end = System.currentTimeMillis();
        System.out.println("RecLinConcat: " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatIc(s,i).length()!=i*l) throw new RuntimeException();
        }
        end = System.currentTimeMillis();
        System.out.println("IterConcat: " + (end-start) + "ms");

        start = System.currentTimeMillis();
        for (int i = 0; i < n; i++) {
            if(repeatSb(s,i).length()!=i*l) throw new RuntimeException();
        }
        end = System.currentTimeMillis();
        System.out.println("IterStrB: " + (end-start) + "ms");
    }

    public static String repeatLog2(String s, int times) {
        if (times <= 0) {
            return "";
        }
        else if (times % 2 == 0) {
            return repeatLog2(s+s, times/2);
        }
        else {
           return s + repeatLog2(s+s, times/2);
        }
    }

    public static String repeatR(String s, int times) {
        if (times <= 0) {
            return "";
        }
        else {
            return s + repeatR(s, times-1);
        }
    }

    public static String repeatIc(String s, int times) {
        String tmp = "";
        for (int i = 0; i < times; i++) {
            tmp += s;
        }
        return tmp;
    }

    public static String repeatSb(String s, int n) {
        final StringBuilder sb = new StringBuilder();
        for(int i = 0; i < n; i++) {
            sb.append(s);
        }
        return sb.toString();
    }
}

It takes 2 arguments, the first is the number of iterations (each function run with repeat times arg from 1..n) and the second is the string to repeat.

So far, a quick inspection of the times running with different inputs leaves the ranking something like this (better to worse):

1. Iterative StringBuilder append (1x).
2. Recursive concatenation log2 invocations (~3x).
3. Recursive concatenation linear invocations (~30x).
4. Iterative concatenation linear (~45x).

I wouldn't ever guessed that the recursive function was faster than the for loop :-o

Have fun(ctional xD).

This contains less characters than your question

public static String repeat(String s, int n) {
    if(s == null) {
        return null;
    }
    final StringBuilder sb = new StringBuilder(s.length() * n);
    for(int i = 0; i < n; i++) {
        sb.append(s);
    }
    return sb.toString();
}

based on fortran's answer, this is a recusive version that uses a StringBuilder:

public static void repeat(StringBuilder stringBuilder, String s, int times) {
    if (times > 0) {
        repeat(stringBuilder.append(s), s, times - 1);
    }
}

public static String repeat(String s, int times) {
    StringBuilder stringBuilder = new StringBuilder(s.length() * times);
    repeat(stringBuilder, s, times);
    return stringBuilder.toString();
}

using Dollar is simple as typing:

@Test
public void repeatString() {
    String string = "abc";
    assertThat($(string).repeat(3).toString(), is("abcabcabc"));
}

PS: repeat works also for array, List, Set, etc

using only JRE classes (System.arraycopy) and trying to minimize the number of temp objects you can write something like:

public static String repeat(String toRepeat, int times) {
    if (toRepeat == null) {
        toRepeat = "";
    }

    if (times < 0) {
        times = 0;
    }

    final int length = toRepeat.length();
    final int total = length * times;
    final char[] src = toRepeat.toCharArray();
    char[] dst = new char[total];

    for (int i = 0; i < total; i += length) {
        System.arraycopy(src, 0, dst, i, length);
    }

    return String.copyValueOf(dst);
}

EDIT

and without loops you can try with:

public static String repeat2(String toRepeat, int times) {
    if (toRepeat == null) {
        toRepeat = "";
    }

    if (times < 0) {
        times = 0;
    }

    String[] copies = new String[times];
    Arrays.fill(copies, toRepeat);
    return Arrays.toString(copies).
              replace("[", "").
              replace("]", "").
              replaceAll(", ", "");
}

EDIT 2

using Collections is even shorter:

public static String repeat3(String toRepeat, int times) {
    return Collections.nCopies(times, toRepeat).
           toString().
           replace("[", "").
           replace("]", "").
           replaceAll(", ", "");
}

however I still like the first version.

Not the shortest, but (i think) the fastest way is to use the StringBuilder:

 /**
   * Repeat a String as many times you need.
   *
   * @param i - Number of Repeating the String.
   * @param s - The String wich you want repeated.
   * @return The string n - times.
   */
  public static String repeate(int i, String s) {
    StringBuilder sb = new StringBuilder();
    for (int j = 0; j < i; j++)
      sb.append(s);
    return sb.toString();
  }

a straightforward one-line solution:
requires Java 8

Collections.nCopies( 3, "abc" ).stream().collect( Collectors.joining() );

If you are worried about performance, just use a StringBuilder inside the loop and do a .toString() on exit of the Loop. Heck, write your own Util Class and reuse it. 5 Lines of code max.

Try this out:

public static char[] myABCs = {'a', 'b', 'c'};
public static int numInput;
static Scanner in = new Scanner(System.in);

public static void main(String[] args) {
    System.out.print("Enter Number of Times to repeat: ");
    numInput = in.nextInt();
    repeatArray(numInput);
}

public static int repeatArray(int y) {
    for (int a = 0; a < y; a++) {
        for (int b = 0; b < myABCs.length; b++) {
            System.out.print(myABCs[b]);                
        }
        System.out.print(" ");
    }
    return y;
}

If you only know the length of the output string (and it may be not divisible by the length of the input string), then use this method:

static String repeat(String s, int length) {
    return s.length() >= length ? s.substring(0, length) : repeat(s + s, length);
}

Usage demo:

for (int i = 0; i < 50; i++)
    System.out.println(repeat("_/‾\\", i));

Don't use with empty s and length > 0, since it's impossible to get the desired result in this case.

Despite your desire not to use loops, I think you should use a loop.

String repeatString(String s, int repetitions)
{
    if(repetitions < 0) throw SomeException();

    else if(s == null) return null;

    StringBuilder stringBuilder = new StringBuilder(s.length() * repetitions);

    for(int i = 0; i < repetitions; i++)
        stringBuilder.append(s);

    return stringBuilder.toString();
}

Your reasons for not using a for loop are not good ones. In response to your criticisms:

1. Whatever solution you use will almost certainly be longer than this. Using a pre-built function only tucks it under more covers.
2. Someone reading your code will have to figure out what you're doing in that non-for-loop. Given that a for-loop is the idiomatic way to do this, it would be much easier to figure out if you did it with a for loop.
3. Yes someone might add something clever, but by avoiding a for loop you are doing something clever. That's like shooting yourself in the foot intentionally to avoid shooting yourself in the foot by accident.
4. Off-by-one errors are also mind-numbingly easy to catch with a single test. Given that you should be testing your code, an off-by-one error should be easy to fix and catch. And it's worth noting: the code above does not contain an off-by-one error. For loops are equally easy to get right.
5. So don't reuse variables. That's not the for-loop's fault.
6. Again, so does whatever solution you use. And as I noted before; a bug hunter will probably be expecting you to do this with a for loop, so they'll have an easier time finding it if you use a for loop.

public static String rep(int a,String k)

       {
           if(a<=0)
                return "";
           else 
           {a--;
               return k+rep(a,k);
       }

You can use this recursive method for you desired goal.

Consolidated for quick reference:

public class StringRepeat {

// Java 11 has built-in method - str.repeat(3);
// Apache - StringUtils.repeat(3);
// Google - Strings.repeat("",n);
// System.arraycopy

static String repeat_StringBuilderAppend(String str, int n) {

    if (str == null || str.isEmpty())
        return str;

    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < n; i++) {
        sb.append(str);
    }
    return sb.toString();
}

static String repeat_ArraysFill(String str, int n) {
    String[] strs = new String[n];
    Arrays.fill(strs, str);
    return Arrays.toString(strs).replaceAll("\\[|\\]|,| ", "");
}

static String repeat_Recursion(String str, int n) {
    if (n <= 0)
        return "";
    else
        return str + repeat_Recursion(str, n - 1);
}

static String repeat_format1(String str, int n) {
    return String.format(String.format("%%%ds", n), " ").replace(" ", str);
}

static String repeat_format2(String str, int n) {
    return new String(new char[n]).replace("\0", str);
}

static String repeat_format3(String str, int n) {
    return String.format("%0" + n + "d", 0).replace("0", str);
}

static String repeat_join(String str, int n) {
    return String.join("", Collections.nCopies(n, str));
}

static String repeat_stream(String str, int n) {
    return Stream.generate(() -> str).limit(n).collect(Collectors.joining());
}

public static void main(String[] args) {
    System.out.println(repeat_StringBuilderAppend("Mani", 3));
    System.out.println(repeat_ArraysFill("Mani", 3));
    System.out.println(repeat_Recursion("Mani", 3));
    System.out.println(repeat_format1("Mani", 3));
    System.out.println(repeat_format2("Mani", 3));
    System.out.println(repeat_format3("Mani", 3));
    System.out.println(repeat_join("Mani", 3));
    System.out.println(repeat_stream("Mani", 3));

}

}